Historically, most ignition systems for internal combustion engines have used a distributor. Distributor type ignition systems generally consist of a single ignition coil to generate high voltage electrical energy and a distributor to mechanically distribute this high voltage electrical energy to the spark plugs within each engine cylinder. As is well known, delivery of the electrical energy is timed so that the spark plugs ignite a fuel mixture charge in each cylinder at or near the beginning of each cylinder compression stroke. The ignition coil used in distributor ignition systems typically has a single primary winding and a single secondary winding, where voltage pulses across the primary winding induce high voltage pulses in the secondary winding. The distributor typically distributes the high voltage pulses using a rotor which rotates to sequentially direct the voltage pulses from the secondary winding to peripherally distributed electrodes in a distributor cap which are, in turn, connected to spark plug wires attached to individual spark plugs.
Recently, distributorless ignition systems have become common. In contrast to distributor type systems, distributorless ignition systems do not have a mechanical distributor and typically use one coil for every two cylinders to generate the high voltage pulses. In most distributorless systems, each end of an ignition coil secondary winding is connected to a spark plug, in contrast to conventional distributor ignition systems in which one end of the coil's secondary winding is attached to the engine ground. The two spark plugs attached to a given coil are in companion cylinders (cylinders that are at top dead center at the exact same time). To cause the spark plugs to fire while the cylinders are at top dead center, a distributorless ignition system electronically senses top dead center and directs a high voltage surge to each coil with no moving parts. One cylinder (referred to as the "event cylinder") will be on the compression stroke at the time of a given firing, while the other cylinder (referred to as the "waste cylinder") is on the exhaust stroke. In a small percentage of distributorless ignition systems, a separate coil is used for each cylinder, one end of each coil being coupled to a cylinder and the other end being coupled to ground. This type of system operates in a manner similar to the more common distributorless ignition systems having one coil for every two cylinders, except each spark plug is fired separately. Distributorless ignition systems generally offer many advantages over conventional mechanical distributor systems, including the advantages of fewer moving parts, more compact mounting, and no unnecessary mechanical load on the engine. Distributorless ignition systems are now well known in the art.
Distributorless ignition systems commonly include an electronic control module which is located in the passenger compartment or under the hood. The control module causes sequential energization of the ignition coils, which are located in one or more housings referred to as coil packs. The coil pack or packs are mounted in close proximity to the engine, and a cosmetic aluminum cover commonly covers all or a portion of the coil packs. More specifically, using switching transistors, the control module controls the coupling of voltage to the primary winding of each ignition coil, each coil generating secondary voltage pulses in response thereto, causing the spark plug to fire.
Many modern distributorless ignition systems, such as the General Motors Quad 4 system, also eliminate the need for secondary spark plug wires. The components of such a system, including the coil packs, are mounted on top of the cylinder head. In place of spark plug wires, such wireless distributorless ignition systems use a fiat wire circuit which is located in the housing to distribute the secondary voltage to the spark plugs. This reduces the capacitance of the secondary circuit, allowing the high voltage pulse generated by the secondary winding of the coil to have a faster rise time. This enables the production of nearly twice as much secondary current as in some mechanical distributor systems.
Because two spark plugs are attached to each ignition coil, the spark plugs fire in a slightly different manner from spark plugs controlled by conventional distributor ignition systems. Electrical current in the secondary circuit passes from the ignition coil through the first spark plug, creating a spark from the spark plug's inner electrode to its outer electrode, as in conventional ignition systems. However, the electrical current then passes through the engine block/ground to the outer electrode of the second spark plug, arcing across to the center electrode (i.e. arcing in the reverse direction from conventional systems) and back to the ignition coil to complete the circuit. Put simply, one spark plug fires forward and the other fires backward.
Due to the differences between distributorless ignition systems and conventional, distributor ignition systems, it is difficult to test a distributorless ignition system using conventional engine analyzer equipment. For instance, there is no common point from which secondary or high voltage waveform signals for all cylinders can be coupled to an engine analyzer for analysis. In addition, there is no common primary circuit from which a primary or low voltage signal can be detected for all cylinders. Finally, spark plug wires, which historically provided convenient locations at which to detect electrical signals, are not present in many distributorless ignition systems. The current industry standard test procedure for testing distributorless ignition systems calls for the technicians to disassemble the ignition system and install spark plug wires. This approach is time consuming, and is also potentially inaccurate due to the installation of spark plug wires on a previously wireless system.
Accordingly, there is a need for a means of testing distributorless ignition systems utilizing conventional automotive testing equipment that can be used on systems having no spark plug wires without requiring that the ignition system be disassembled or spark plug wires installed.